Remove the Office file placeholder that only showed "Preview will be
supported in a future update" without any download option. Office files
(pdf, doc, docx, xls, xlsx, ppt, pptx) now fall through to the generic
"unsupported file" handler which provides a download button.
Removed:
- OfficeFilePlaceholder component
- isOfficeFile function and OFFICE_EXTENSIONS constant
- isOffice flag from useFileTypeInfo hook
- i18n keys for officePlaceholder
This simplifies the file type handling to just three categories:
- Editable: markdown, code, text files → editor
- Previewable: image, video files → media preview
- Everything else: download button
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